1. Field of Invention
The present invention relates to a method fore manufacturing a low thermal-impedance insulated metal substrate, and more particularly to a method for manufacturing a low thermal-impedance insulated metal substrate with improved lower thermal-impedance and higher electrical reliability to enhance higher life span of an electronic device with the low thermal-impedance insulated metal substrate. The low thermal-impedance insulated metal substrate provides lower thermal expansion the same.
2. Description of the Related Art
Electronic devices are developed to have high efficiency, so more power is required to drive electronic devices. When an electronic device is in operation, heat will be generated from the electronic device and accumulated in the electronic device, damaging the electronic device and shortening a life span and electrical reliability of the electronic device if the heat cannot be dissipated. For example, light emitted diodes (LED) are used as back light units or the like. Especially in the lighting industry, LEDs are actively used to replace incandescent lamps which increase commercial demand for LEDs. However, only 15˜25% of electricity input can be converted into light in LEDs and other electricity input is converted into heat. Therefore, the heat accumulates in the LED, which causes decrease of luminous intensity, shortening of life span of the LED, light emitted color-shift and yellowing of packaging or the like.
With reference to FIG. 5, in order to solve the foregoing disadvantages, an electronic element (40) can be mounted on an insulated metal substrate (30) to dissipate heat from the electronic element (40) to a thermal module. A common insulated metal substrate (30) has an electrical-conductive metal layer (31), a thermal-conductive metal layer (33) and an insulation layer (32) with thermal-conductive and adhesive functions between the electrical-conductive metal layer (31) and the thermal-conductive metal layer (33). Generally, there are three conventional methods for manufacturing the insulated metal substrate (30).
The first conventional method comprises steps of mixing inorganic thermal-conductive filler and thermoplastic resin to form a composite solution; coating the composite solution on a surface of the electrical-conductive metal layer (31) and a surface of the thermal-conductive metal layer (33); drying the electrical-conductive metal layer (31) and the thermal-conductive metal layer (33) to form two thermoplastic thermal-conductive composite layers respectively on the electrical-conductive metal layer (31) and the thermal-conductive metal layer (33); adhering the thermoplastic thermal-conductive composite layers of the electrical-conductive metal layer (31) and the thermal-conductive metal layer (33) to form a thermoplastic thermal-conductive composite layers (31, 33); melting the thermoplastic thermal-conductive composite layers (31, 33) by the thermal compressing process, so that the layers (31, 33) can be adhered and combined to form an insulation layer (32) between them and the insulated metal substrate (30) is obtained. However, the process of thermal compression requires a temperature higher than 200° C., and voids or pores are easily generated in interfaces between layers, which increases thermal-impedance of the insulated metal substrate (30).
The second conventional method comprises steps of mixing inorganic thermal-conductive filler and liquid thermosetting resin to form a slurry; coating the slurry on the thermal-conductive metal layer (33) to form a thermal-conductive composite layer; covering the electrical-conductive metal layer (31) on the thermal-conductive composite layer; curing the thermal-conductive composite layer by the hot-press process to form an insulation layer (32). However, since the slurry is liquid state before curing, high temperature and pressure process may cause the resin-flow phenomenon, which the slurry spills out of the layers (31, 33). During manufacturing process, the inorganic thermal-conductive filler and thermosetting liquid resin may be separated into inhomogeneous. Therefore, the insulation layer (32) has poor thermal conductivity and electrical reliability.
The third conventional method comprises steps of blending inorganic thermal-conductive filler, thermoplastic resin and thermosetting resin at a temperature higher than melting points of the resins to form a rubber; adding thermosetting epoxy curing agent and catalyst into the rubber; extruding, calendering, injection molding the rubber and a releasing material to form a thermal-conductive insulating composite layer with the releasing substrate; removing the releasing substrate; putting and pressing the thermal-conductive insulated composite layer between the electrical-conductive metal layer (31) and the thermal-conductive metal layer (33) at increased temperature, which is served as an insulation layer (32); and obtaining the insulated metal substrate (30). After curing process, the resins of the thermal-conductive insulated composite layer form the inter-penetrating network structure. However, to melt the thermoplastic resin requires higher temperature and the inorganic thermal-conductive filler is difficult to be dispersed homogeneously in a high viscosity fluid thermoplastic resin. During the pressing process of the thermal-conductive insulating composite layer, voids or pores are easily formed at interface between each two layers to increase thermal-impedance of the insulated metal substrate (30).
For achieving a suitable electrical reliability, each insulated layer (32) of insulated metal substrate (30) produced by the foregoing methods has a thickness larger than 75 μm. Besides, in order to lower the thermal-impedance, thermal-conductivity of each insulated layer (32) should be raised. Therefore, the inorganic thermal-conductive filler is more than 50 vol. % of the insulated layer (32), which results in decreasing mechanical properties, so the insulated layer (32) will be easily creaked to decrease the electrical reliability.
To overcome the shortcomings, the present invention provides a method for manufacturing a thermal-conductive substrate with lower thermal-impedance to mitigate or obviate the aforementioned.